As shown in FIG. 1, a manhole 12 is closed by a manhole cover 11 on the ground surface. Sometimes, the ground surface will be further paved with asphalt. In this case, as the manhole cover 11 may not be flush with the road surface, either road bump or depression would be formed. Traffic accidents may thus occur.
To solve this problem, as shown in FIG. 2, a manhole opening 12A is arranged at a depth (about 25 cm to 30 cm) beneath the ground surface, and a pavement 13 is paved on the manhole cover 11. In this case, to access to the manhole 12, it is necessary to locate the manhole cover 11. Taiwanese patent no. 1398567 discloses a method for locating a manhole cover using a metal detector.
Taiwanese utility module patent no. M507942 discloses another method for locating a manhole cover. Two radio-frequency identification (RFID) tags are disposed diagonally opposite to each other on the manhole cover. A center of the manhole cover beneath the ground surface may be determined using the two RFID tags.